The overall objective of this proposal is to define the role of estrogen in the growth of 7,12-dimethylbenz(a)anthracene (DMBA)-induced rat mammary tumors. The following specific goals are set for the period proposed: (1) The method of our current in vitro incorporation of H3-leucine by the tumor tissue will be modified to increase the sensitivity of the test by reducing isotopic dilution with unlabeled leucine in the incubation medium. (2) The previous finding of synergistic enhancement of H3-leucine incorporation in the tumor by estrogen and prolactin will be further confirmed. The hypothesis is that the amount of estrogen receptor is the limiting factor in the response of rat mammary tumors to estrogen and that prolactin may alter the tissue response by increasing the amount of this receptor. (3) Mammary tumors which do not regress after ovariectomy will also be analyzed for the presence of estrogen receptor and the possible autogenous synthesis of estrogen. In this sense, an "autonomous" tumor may still be estrogen-responsive, despite the fact it is able to survive in the absence of host's ovaries. The term "ovarian independence" should be adopted, if this is the case.